Flux pinning and creep in high- Tc superconductors

The theory of collective pinning in the presence of thermal fluctuations of vortex lines is developed. Critical current j_c is shown to decrease rapidly with temperature. Flux creep under the action of weak ( j << j_c) current is considered and j-dependence of the effective energy barrier U( j) is estimated. At the current decrease U( j) grows initially as j^{- α}, and then saturates at a finite value U_pl, due to plastic deformations coming into play. The relations is discussed between this theory and the existing experiments.